The relativistic mass increase is rather insignificant according to my calcs (about factor of 1.000000039), so perhaps the explanation is that the craft will travel far enough from Sun, and the flow of photons will be too thin? Let's do the math:

How much time it will take to accelerate from 0 to 300,000km/h at a constant acceleration of 1mm/ss? That would be

That's about as far as the boundary of the Solar System. I am speculating that there are not enough photons that far from the Sun to hit the sail.

Interesting concept, though -- I remember seeing an executive toy -- a glass sphere, vacuum inside, and the mill constanly in motion, propelled just by the pressure of the outside light. [ June 08, 2005: Message edited by: John Smith ]

300.000 kmh is the speed of light (approximately, it's an oft used approximation) which is the speed limit of this universe (and if you were to go faster you'd go faster than the stuff that's powering your engine...).

Given John's calculations, it would take some 3 years to reach this speed. Of course over that time the accelleration would decrease as the light pressure decreases due to the increased dispersion of the photons (remember the sun at such distances is effectively a point source radiating in all directions so the percentage of the photons a solar sail with constant surface area catches decreases over time).

300.000 kmh is the speed of light (approximately, it's an oft used approximation)

More precisely, 300.000 km/h is 1/3598 of the speed of light. I think you mixed kmh amd m/s.

Jeroen Wenting
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posted Jun 09, 2005 07:02:00

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hmm, my bad

But given that it takes 3 years to achieve that speed and by then it will be far far away and the solar light pressure extremely low that's the reason it won't go faster. Using launching lasers you could get a far higher speed, probably up to 10+% lightspeed easily, maybe even faster.

Achieving true relativistic speeds might be impossible because even launching lasers would likely not project enough energy onto a solar sail to keep accellerating the craft after a point (because of the ever greater energy needed when the mass of the craft goes up due to relativistic effects).

Maybe a combination drive consisting of solar sails and mass drivers with electromagnetic scoops (to collect fuel from deep space) might be the thing. But even those won't get you to really high fractions of lightspeed.

isn't the speed of light a boundary, not a limit? i thought i've read (possibly on another MD thread) that things going SLOWER than light cannot accelerate to it, but thing ALREADY travelling FASTER than light can indeed accelerate even more, but cannot slow down below that speed...

There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors

Of course all this is just theory, but yes you can travel up to light speed and if you are traveling faster than light speed you can accelerate even faster, but you cannot travel at light speed.

So in order to travel faster than light speed you must find a way to instintaniously accelerate from slower than light speed to faster than light speed without actually hitting light speed.

Jeroen Wenting
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posted Jun 10, 2005 00:29:00

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correct. It's a not so much a limit as it is a discontinuity. Mass rises exponentially as speed goes up until lightspeed is reached. From that point on mass drops exponentially.

Thus an object approaching lightspeed from below will experience ever decreasing accelleration (unless it has infinite power to accellerate which is increased constantly to maintain constant accelleration) until lightspeed is reached (which in practice is thus impossible because accelleration will stop before reaching lightspeed). Once breaking the lightbarrier accelleration will then increase constantly as mass drops with increasing speed. On slowing down from superluminal speeds the reverse is true. Decelleration (with finite power) becomes ever harder as the lightbarrier is approached and mass increases, then becomes ever easier after reverting to subluminal speed as mass decreases again.

I don't thing the theoretical limit of the speed of a solar sail has anything to do with the lack of photons to move you along. I think they assume that there are plenty of photons available. My understanding is that somewhere between 10% and 25% of the speed of light, the solar sail is running into so many particles at such high relative speeds that the friction with those particles prevents it from going any faster.

My understanding is that somewhere between 10% and 25% of the speed of light, the solar sail is running into so many particles at such high relative speeds that the friction with those particles prevents it from going any faster.

Well, acording to the specifications in the first post, the maximum speed is only 1/3598 (0.027%) of the speed of light. And how does the friction come in play if the sail is moving away from Sun? Or do you mean some other particles going in the opposite direction, hitting the sail?

Thomas Paul
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posted Jun 10, 2005 20:14:00

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Originally posted by John Smith: Well, acording to the specifications in the first post, the maximum speed is only 1/3598 (0.027%) of the speed of light. And how does the friction come in play if the sail is moving away from Sun? Or do you mean some other particles going in the opposite direction, hitting the sail?

He is wrong about the theoretical speed limit for solar sails. The estimates range from 10% to 25% of the speed of light. The particles I am talking about are the particles in space. Space is not empty. It is full of matter. At slow speeds that is irrelevant but at faster speeds you are encountering more and more particles per second with more energetic collisions.

Jeroen Wenting
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posted Jun 11, 2005 03:02:00

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You may be right, but the reduced amount of light pressure as the craft moves further away from the primary will reduce the accelleration. At some point the light pressure will no longer be enough to further accellerate the mass of the craft against the impact of interstellar dust. Maybe my idea of adding an EM scoop and massdriver isn't so farfetched after all. You would get a lightsail to accellerate the craft to a speed where the scoop can operate and then use that. As you near your destination redeploy the sail and use the drag to slow you down without having to turn the ship around and let the massdriver brake you.

One thing that has always struck me as being inaccurate in sci-fi films/programmes is the huge range of destinations for the space ships to zoom around. You would have thought that the percentage of star systems out there that have a planet of the right size, with the right atmosphere and the right climate for humans to live on would be very very small. This should mean that most space ships have to fly around for decades to get to the next Earth-like planet, but in most sci-fi films/programmes there are huge federations/empires/republics etc filled with hundreds of nice planets.

Of course accurate "hard" sci-fi probably doesn't seem as exciting to the people funding it, so we end up with a more fantasy based sci-fi. Perhaps this is why the vast majority of sci-fi programmes with inter-stellar transport rely on a kind of transportation Deus-Ex-Machina in the form of Warp/Worm holes/Hyperspace and so on. It would be nice to see one that attempts to have some form of realistic science behind it.

There will be glitches in my transition from being a saloon bar sage to a world statesman. - Tony Banks

Jeroen Wenting
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posted Jun 13, 2005 05:30:00

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I know. Of course we don't KNOW how many earthlike planets there are out there, nor how many of those have an atmosphere (and associated fauna and flora, as it is highly unlikely an oxygen based atmosphere would develop without carbon based flora) that supports us.

There is (or was) a science fiction series that deals with this, it's BBCs Red Dwarf. While of course not to be taken seriously (after all, it's a comedy) it does deal with long space travel at subluminal speeds.

I didn't mean to allude "destination" as being nearby, it can be anywhere. In my mind it doesn't have to be a system with earthlike planets either. Once we start to settle space and embark on interstellar journeys I envision us leaving planetary surfaces behind as our main living quarters and moving into large space settlements instead. Far easier without the burden of a steep gravity well to overcome.

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posted Jun 15, 2005 02:51:00

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I wonder if we'll ever send people on long distance extra-solar journeys. Given the huge amounts of time people will spend on board these space ships, in order to reach somewhere worth sending people to, the ship would probably have to either have a hibernation or a generation (i.e. the ship takes hundreds of people and goes through several generations en-route) system.

Neither of these seem particularly likely though. Hibernation technology seems a long way off, and a generational approach would require a massive ship. Would either of these be economically feasible? Would any government or corporation be willing to pay the stupendous amounts needed to send people to another solar system? Perhaps the only extra-solar travellers in the foreseeable future will be mechanical probes.

Obviously this could change if we found some magical new propulsion, medical, radiation shielding, food growing, air recycling and water cleaning systems, but the most likely scenario seems to be that we're stuck in this solar system for the time being [ June 15, 2005: Message edited by: Dave Lenton ]

Jeroen Wenting
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posted Jun 15, 2005 05:40:00

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Using space settlements of tens of thousands of people (which I see happening relatively soon, they're the logical next step, far more economical than colonising other planetary surfaces) generation ships are certainly a very real possibility.

A space settlement of say 50.000 people can feature a self-sufficient ecosystem and industry and would not need a great investment to turn into a self-propelled spaceship. Some massdrivers to kick it out of orbit, a few more to provide steering, and you're almost there. It doesn't even have to be fast, who cares if you can go at only 5-10% lightspeed? The trip would be little different (apart from the widening communications gap and the lack of outside visitors) from circling in the asteroid belt or in HEO.

Such a station wouldn't require a habitable planet at the star it's going to, all it would need is either an asteroid belt to provide raw materials for more settlements to start colonising the system or (if the system isn't suitable) some source of raw materials to create fuel to move on (either comets or maybe a gas giant would suffice).

Once such missions have succeeded in establishing human colonies among distant stars the urge to visit Sol and Earth would cause a massive drive towards the development of faster and fuel efficient alternatives (possibly superluminal propulsion).

Thomas Paul
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posted Jun 15, 2005 20:31:00

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Originally posted by Dave Lenton: Of course accurate "hard" sci-fi probably doesn't seem as exciting to the people funding it, so we end up with a more fantasy based sci-fi. Perhaps this is why the vast majority of sci-fi programmes with inter-stellar transport rely on a kind of transportation Deus-Ex-Machina in the form of Warp/Worm holes/Hyperspace and so on. It would be nice to see one that attempts to have some form of realistic science behind it.

Larry Niven wrote some realistic stories about the problems of colonizing space when you can only travel at sub-light speeds.

Looks like it was built with the private American funds by a Russian spacecraft contractor and was launched from the tip of a intercontinental ballistic missile off the Russian submarine. [ June 21, 2005: Message edited by: John Smith ]

it is strange that i was doing very well in physics and related subject from junior middle school in China all the way to graduate school in the US, but did not have much interest ever since i graduated.